KR20010049219A - Liquid crystal alignment layer, liquid crystal display device having the same, and composition for forming the layer - Google Patents

Abstract

PURPOSE: A liquid crystal alignment film, an alignment film compound and a liquid crystal display device are to reduce a residual image and improve a contrast ratio of the liquid crystal display device. CONSTITUTION: The liquid crystal display device comprises a pair of substrates parallel to each other, a thin film electrode formed on the substrates and a liquid crystal alignment film. A liquid crystal alignment film compound includes a polyimide component and a polyamic acid. The polyamic acid is distributed toward the substrates, and the polyimide is distributed in a direction opposite to the substrates. The alignment film is formed with a methylene diamine(MDA) and a para-phenylene diamine(p-PDA). The alignment film has an angle variation ranged from -1 degree to +1 degree when a file contact length is changed from 8 millimeter to 14 millimeter.

Description

Liquid crystal alignment film, alignment film composition and liquid crystal display device {LIQUID CRYSTAL ALIGNMENT LAYER, LIQUID CRYSTAL DISPLAY DEVICE HAVING THE SAME, AND COMPOSITION FOR FORMING THE LAYER}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal alignment film composition, a liquid crystal alignment film formed by the same, and a liquid crystal display device, and more particularly, to a liquid crystal alignment film composition and an alignment film for manufacturing a low voltage driving liquid crystal display device having a large contrast ratio and low afterimages. It is about.

Liquid crystal display, one of the representative flat panel display devices, injects liquid crystal between two thin glass plates coated with a conductive film, and then applies an electric field between the conductive films through a driving IC to control the arrangement of liquid crystal molecules, thereby transmitting light. To display an image. Such liquid crystal display devices are classified into simple matrix and active matrix methods according to the driving method, and are now active matrix type thin film transistor liquid crystal display (TFT-) with high image quality and fast response speed. LCD) is widely used. Since the liquid crystal display is mainly used for a portable device such as a notebook, it needs to consume as little power as possible, and has a problem of low resolution and afterimage remaining as compared to other display devices. It is going on.

Since the driving threshold voltage of the liquid crystal is inversely proportional to the square of the dielectric anisotropy Δε of the liquid crystal, the dielectric anisotropy Δε should be increased to lower the driving voltage of the liquid crystal. However, if the dielectric anisotropy Δε is increased, the viscosity of the liquid crystal rises, and the response speed is slowed, and also, afterimage is easy to occur because it easily adsorbs the surrounding ions due to the increase in the polarity of the liquid crystal compound. As described above, studies are being actively conducted to solve various problems caused by raising the Δε of the liquid crystal by changing the chemical structure of the alignment layer having the closest interaction with the liquid crystal.

Nissan Co., Ltd. et al. Have disclosed that the residual DC decreases when the polarity of the diamine in the polyimide is decreased and the voltage retention of the alignment film is increased. T. Sato et al. Reported that polyimide is advantageous in terms of voltage retention and that polyamic acid is effective in reducing residual DC. As described above, various hypotheses about the role of the alignment layer on the contrast ratio and the afterimage phenomenon of the liquid crystal display have been raised. However, it is not known exactly what properties of the alignment layer affect the quality of the liquid crystal display.

In the liquid crystal display device, the factor that determines the contrast ratio depends on how much light leakage in the black state is prevented, and thus, it is determined that the pretilt angle uniformity of the alignment layer has the greatest influence on the contrast ratio. In addition, the afterimage is affected by the resistivity of the liquid crystal and the peripheral material. However, in the case of a low voltage liquid crystal, since Δε is large, the afterimage is very vulnerable to ion impurity in the peripheral area, resulting in an afterimage. Role is very important.

Accordingly, it is an object of the present invention to provide a liquid crystal alignment film which not only has low adsorption power for ionic impurities generated in liquid crystals and peripheral materials, but also easily releases adsorbed ions, and a liquid crystal alignment film composition for preparing the liquid crystal alignment film. Another object of the present invention is to provide a liquid crystal alignment film and a liquid crystal alignment film composition for producing a liquid crystal display device with less residual image.

The present invention also provides a liquid crystal alignment film and a liquid crystal alignment film composition which can improve the uniformity of the pretilt angle to improve the contrast ratio of the liquid crystal display device.

BRIEF DESCRIPTION OF THE DRAWINGS The schematic diagram for demonstrating formation of the residual DC component in a liquid crystal display device.

Figure 2 is a schematic diagram showing a shape coated on the substrate the alignment film containing both polyimide and polyamic acid according to the present invention.

3A and 3B are schematic views of (a) when an alignment film is prepared using an alkylside chain type diamine and (b) when an alignment film is prepared using an alkyl main chain type diamine, respectively.

4 is a graph showing the change in the pretilt angle according to the pile contact length when the alkyl main chain type and the alkyl site chain type are used as the pretilt angle expression unit, respectively.

5 is a graph illustrating a contrast ratio of a liquid crystal display device to which an alignment layer according to a comparative example and an embodiment is applied.

6 is a graph illustrating an afterimage phenomenon of a liquid crystal display device to which an alignment layer according to a comparative example and an embodiment is applied.

In order to achieve the above object, the present invention provides a liquid crystal alignment film composition comprising a copolymer and a solvent containing both a polyimide component and a polyamic acid component. In another aspect, the present invention is a liquid crystal alignment film formed on a substrate, including both a polyimide component and a polyamic acid component, wherein the polyamic acid component is distributed on the substrate side, the polyimide is distributed on the opposite side of the substrate A liquid crystal display device comprising an alignment layer is provided.

The copolymer is prepared by polymerizing a dianhydride compound and a diamine compound, preferably, by dividing the dianhydride component into two or more stages to polymerize the diamine component, and the diamine component is free of alkyl main chain type. It is preferable that the tilt each expression unit is included.

Hereinafter, the present invention will be described in detail.

Since the residual image phenomenon of the liquid crystal display device is due to the residual DC component generated inside the cell, the electrical characteristics of the alignment layer in contact with the liquid crystal are closely related to the afterimage phenomenon. That is, as the polarization of the alignment layers 10 and 10 ′ itself is larger, as shown in FIG. 1, the ionic components 30 in the liquid crystal 20 are more adsorbed and aligned to increase the residual direct current in the cell. In Fig. 1, reference numerals 40 and 40 'denote upper and lower electrodes, respectively, and reference numerals 50 and 50' denote upper and lower glass substrates, respectively.

Therefore, there is a demand for a method capable of suppressing the residual DC voltage generated inside the cell by the design of the alignment layer, but the residual DC generation mechanism cannot be explained by a model of the specific resistance and dielectric constant of the alignment layer. It is estimated that the polarization of the alignment film due to the application of direct current voltage and the ion drop at the alignment film / liquid crystal interface contribute.

Polyamic acid of the general formula (1), which is generally used as an alignment layer at present, has good printability and mass productivity, but since the polyamic acid has many polar groups such as -COOH on the surface of the alignment layer in contact with the liquid crystal, the alignment layer itself is easy to polarize, Increase the residual direct current component. This problem becomes more problematic in recent years because liquid crystals having a large Δε are used to lower the driving of the liquid crystal.

Therefore, in the present invention, by using an alignment film in which polyimide and polyamic acid are copolymerized, the problem of afterimages, which are more vulnerable while using low voltage liquid crystal, is not solved without lowering the printability of the alignment film. For example, the structural formula of the alignment film component of the polyimide and polyamic acid copolymerized form of the present invention is represented by the following formula (2).

The alignment layer including both the polyimide and the polyamic acid of the present invention is in the form of a copolymer in which the polyimide and the polyamic acid coexist in one polymer. When coating, the probability of going to the upper layer (the opposite side of the substrate) increases, and since polyamic acid is hydrophilic (water-loving property), there is a high probability of facing toward the substrate. Therefore, when the alignment film is coated on the substrate, the hydrophilic polyamic acid has a high probability of being located on the substrate side, that is, the lower side, and the hydrophobic polyimide has a high probability of being located on the upper side, so that the alignment film is naturally located. Therefore, as shown in FIG. 2, polyimide (PI) is positioned in the upper layer (the opposite side of the substrate), and polyamic acid (PA) is positioned in the lower layer (substrate side). The afterimage phenomenon can be suppressed by preventing the adsorption of ionic impurities, and the polyamic acid (PA) located in the lower layer has a large number of -COOH groups, thereby improving the printability of the alignment film.

A preferred method for producing an alignment film containing both the polyimide and polyamic acid of the present invention is first to polymerize a diamine component and an acid anhydride component in a solvent to form a polyamic acid, and then add an acid anhydride component to further polymerize the reaction. It is to let. When the two-stage polymerization reaction is carried out as described above, part of the polyamic acid polymerized by the first polymerization reaction is converted into polyimide during the second polymerization reaction, so that the formed polymer has a structure in which polyimide and polyamic acid coexist. When the alignment agent thus formed is applied to the substrate, as shown in FIG. 2, the polyamic acid portion PA in the alignment layer component is predominantly bonded to the substrate, which is about 30 to 30 ° C., preferably 180 to 220 ° C. When heated for a minute to 90 minutes, preferably about 1 hour, the solvent is evaporated and converted to a polyimide from a polyamic acid located on the outside of the substrate to form a complete alignment film. In the process of forming the alignment layer, it is also possible to have a polyamic acid that is not changed to polyimide on the substrate side.

As the diamine used in the preparation of the alignment agent, all diamines commonly used in the preparation of the alignment layer may be used. Preferably, methylene diamine (MDA) of Formulas 3 and 4 and P-phenylenediamine ( para-phenylene diamine: p-PDA) can be used.

The acid anhydride component may also be used all materials commonly used in the preparation of the alignment layer, preferably a compound of formula 5 (cyclobuthane dianhydride: CBDA) to 7 can be used.

In addition, as a solvent, without limitation, gamma-butyrolactone, N-methylpyrrolidone, butyl cellosolve, or the like may be used alone or in combination. -Butyrolactone: N-methylpyrrolidone: Butyl cellosolve is preferably mixed in a volume ratio of 70 to 90: 5 to 20: 5 to 10, more preferably 78: 16: 6 ratio.

The method for producing a copolymer containing polyimide and polyamic acid can also be used in a wide range of conventional copolymerization methods, and the polymerization temperature and time can be used for the diamine, acid anhydride component, and amount of the desired imide component in the copolymer. It may be variously changed depending on the like.

The present invention is also characterized by further adding a diamine compound in the form where each pretilt expression unit is an alkyl main body in order to improve the contrast ratio of the liquid crystal display device. It is advantageous to lower. The amount of the diamine compound in the alkyl main form to be used is preferably 5 to 15 mol% of the diamine compound for preparing the alignment film polymer component, the diamine compound in the alkyl main chain form to have a structure of the formula (8).

NH ₂ -X-COO-R-COO-X-NH ₂

Wherein X is a substituted or unsubstituted aromatic ring, in particular a benzene group, and R is a straight alkyl having 10 to 20 carbon atoms or a substituted or unsubstituted benzene group.

3 is a schematic diagram of (a) when the alignment film is prepared using an alkylside chain type diamine and (b) when the alignment film is prepared using an alkyl main chain type diamine, wherein the alkyl side chain type diamine is NH ₂ -R '. -NH ₂ (where R 'is a branched alkyl group), so that one end of the pretilt-expressing unit R' 80 is separated from the polymer body 20 as shown in FIG. While the uniform orientation cannot be obtained, -X-COO-R-COO-X- (80), which is a pretilt expression unit of the alkyl main chain type diamine, has a sock end coupled with the polymer body 90, A uniform alignment state can be obtained.

The alignment layer composition is coated on a pair of substrates coated with a thin film electrode by a spin coating method, and dried to form a liquid crystal alignment layer, and then a liquid crystal is filled between the two substrates to manufacture a liquid crystal display device. The glass transition temperature of the alignment film thus formed is 350 ° C or higher and the decomposition behavior is 360 ° C or higher.

Non-limiting examples of the present invention are described below.

[Examples and Comparative Examples]

In order to understand the contrast ratio and the afterimage effect by the alignment film, an alignment film composition was prepared using five polymers as shown in Table 1.

Chemical composition Pre-Tilt Expression Unit Chemical structure Comparative Example 1 p-PDA / MDA / CBDA Crosslinking type Y Polyamic acid Comparative Example 2 MDA / CBDA Z Polyamic acid Example 1 MDA / CBDA Z Polyamic acid / polyimide Example 2 p-PDA / MDA / CBDA Y Polyamic acid / polyimide Example 3 p-PDA / MDA / CBDA Y Polyamic acid / polyimide

In Table 1, MDA is methylenediamine of formula 3, p-PDA is P-phenylenediamine of formula 4, CBDA is cyclobutane dianhydride of formula 5, and Y of the pretilt expression unit is an alkyl main chain type diamine. NH ₂ -X-COO-R-COO-X-NH ₂ (where X is a benzene group and R is a trifluoromethylbenzene group) is mixed, and Z is NH ₂ , an alkylside chain type diamine. -R'-NH ₂ , where R 'is C ₅ H ₁₁ CH. In the chemical structure, the polyamic acid indicates an alignment film composition having only polyamic acid by one-step polymerization, and the polyamic acid / polyimide indicates an alignment film composition including both polyamic acid / polyimide by two-step polymerization. As the liquid crystal used for measuring the contrast ratio and the afterimage effect by the alignment film, a 2.5V low voltage liquid crystal ZSM5136 (ΔIS ≒ 9.6) having a large Δε was used. In addition, the compound of Comparative Example 1 was formed as an alignment film having a crosslinked structure to which a photosensitive group was added to increase the strength of the film and to lower the residual DC.

In order to evaluate the characteristics, the five alignment layer compositions were coated on the substrate at about 700 to 800 Pa, and then fired for 200 ° C./1 hr to form an alignment layer, thereby manufacturing a liquid crystal display test cell.

a) measuring the contrast ratio

The contrast ratio is the ratio of the black luminance to the white luminance. For this value to be improved, a uniform alignment state of the liquid crystal is required. Therefore, if the amount of change in the pretilt angle of the alignment film is large due to various physical damages of the alignment film (particularly due to the rubbing of the alignment film) or the characteristics of the alignment film, light leakage is caused in the black state, so that a good contrast ratio cannot be obtained.

To test this change in contrast ratio and pretilt angle, divide the pile contact length of the cell (the size of the marks and the rubbing strength when rotating the rubbing roll on the substrate) to 8-14 mm. Changes in contrast ratio and pretilt angle were measured. In FIG. 4, changes in pretilt angles according to pile contact lengths of the alignment layers of Comparative Examples 2 and 3 using alkylmain chain types and alkylsite chain types as pretilt expression units were measured.

As shown in FIG. 4, since the alignment layer using the alkylside chain type diamine (Comparative Example 2) has a structure in which the pretilt expression unit is easily movable in space, the alignment uniformity is different depending on the strength and weakness of external stimuli such as rubbing. Since it appears, it is understood that the fluctuation of the tilt angle is large according to the rubbing intensity and the black luminance is lowered. On the other hand, the alignment film (Example 3) having a structure in which the pretilt expression unit is difficult to move spatially is able to maintain a constant pretilt angle even when the rubbing intensity increases, indicating that the luminance decreases little in the black state. . In addition, if the change in the pretilt angle is small according to rubbing, the process margin is widened, which is very advantageous in terms of process.

In FIG. 5, the contrast ratio of the liquid crystal display device to which the alignment films of Comparative Examples and Examples were applied was evaluated 10 times and the average value thereof was shown. As shown in FIG. 5, it can be seen that when each pretilt expression unit is an alkyl main chain type (Comparative Examples 1 and 2 and 3), the contrast improvement is about 30 to 50% or more than that of the alkyl side chain. have.

b) Residual Image

In order to evaluate afterimages and optical characteristics, the alignment film was rubbed under the conditions of a pile contact length of 11 mm, a table speed of 50 mm / sec ² , a roller rotation speed of 1000 rpm, and two rubbing times. In addition, the optical evaluation was calculated by measuring the transmittance of the black / white luminance with BM-5A (Topcon), the afterimage was driven in a test pattern for 14 hours at room temperature, and the change was quantified by measuring the transmittance with BM-5A. , And the results are shown in FIG.

As shown in FIG. 6, it can be seen that an afterimage of the liquid crystal display device using the alignment films of Comparative Examples 1 and 2 is more than twice as large as that of the liquid crystal display device using the alignment films of Examples 1 to 3. Therefore, since the alignment film manufactured using only polyamic acid has a low imidation ratio (50 to 60%), many polar groups such as -COOH are present near the interface in contact with the liquid crystal, and thus ionic impurities are applied when voltage is applied. Is attracted more strongly, and the adsorbed ions accumulate on the surface of the alignment film and remain undischarged (residual DC) even when no voltage is applied (residual DC). On the other hand, since the double-structured alignment films of Examples 1 to 3 contain polyimide having a high imidation ratio on the upper side, the effect of the polyamic acid on the surface of the alignment layer is relatively small, and thus it is judged to have a good result on the residual image side ( There is no afterimage occurring below the dotted line in FIG. 6).

Therefore, the alignment layer of the present invention has a low adsorption force on ionic impurities generated in the liquid crystal and the peripheral material, thereby reducing the afterimage and increasing the uniformity of the rubbing angle, thereby improving the contrast ratio of the liquid crystal display device. In addition, the alignment layer of the present invention includes both the polyimide portion exposed to the upper portion and the polyamic acid attached to the substrate, and part of the polyamic acid is imidized directly on the substrate, so that it is excellent in printability and adhesion to the substrate. Hardness is good.

Claims (10)

A copolymer comprising both a polyimide component and a polyamic acid component; And a liquid crystal alignment film composition comprising a solvent. The liquid crystal aligning film composition of claim 1, wherein the copolymer polymerizes the dianhydride compound and the diamine compound and divides the dianhydride component into two or more steps to polymerize the dianhydride compound. The liquid crystal alignment layer composition of claim 2, wherein the dianhydride compound is a compound of Formula 5, and the diamine compound is a compound of Formula 2 or 3 below. [Formula 5] [Formula 3] [Formula 4] The liquid crystal alignment film composition as claimed in claim 2, wherein the diamine component contains pretilt expression units of alkyl main chain type. The diamine component of claim 4, wherein the diamine component is NH ₂ -X-COO-R-COO-X-NH 2, wherein X is a substituted or unsubstituted aromatic ring, and R is a straight alkyl group having 10 to 20 carbon atoms or substituted And an unsubstituted benzene group.). The method of claim 1, wherein the solvent Butyrolactone: N-methylpyrrolidone: Butyl cellosolve is a liquid crystal alignment film composition in which the volume ratio is mixed in 70 to 90: 5 to 20: 5 to 10. A liquid crystal aligning film formed on a substrate, comprising both a polyimide component and a polyamic acid component, wherein the polyamic acid component is distributed on the substrate side, and the polyimide is distributed on the opposite side of the substrate. 8. The liquid crystal alignment film as claimed in claim 7, wherein the alignment layer contains pretilt expression units of alkyl main chain type. The liquid crystal alignment layer of claim 7, wherein the alignment layer has a change in pretilt angle of ± 1 ° when the pile contact length varies from 8 to 14 mm. A pair of substrates disposed in parallel to each other; A thin film electrode formed on the substrate; A liquid crystal alignment layer formed on the thin film electrode and including both a polyimide component and a polyamic acid component, the polyamic acid component distributed on a substrate side, and the polyimide distributed on a side opposite to the substrate; And And a liquid crystal filled between the liquid crystal alignment layers.